Method of reducing redox ratio of molten glass and the glass made thereby

ABSTRACT

A soda-lime-silica glass for solar collector cover plates and solar mirrors has less than 0.010 weight percent total iron as Fe 2 O 3 , a redox ratio of less than 0.350, less than 0.0025 weight percent CeO 2 , and spectral properties that include a visible transmission, and a total solar infrared transmittance, of greater than 90% at a thickness of 5.5 millimeters, and reduced solarization. In one non-limiting embodiment of invention, the glass is made by heating a pool of molten soda-lime-silica with a mixture of combustion air and fuel gas having an air firing ratio of greater than 11, or an oxygen firing ratio of greater than 2.31. In another non-limiting embodiment of the invention, streams of oxygen bubbles are moved through a pool of molten glass. In both embodiments, the oxygen oxidizes ferrous iron to ferric iron to reduce the redox ratio.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method of reducing the redox ratio(FeO/Fe₂O₃) of molten glass, and the glass made thereby, and moreparticularly, to a method of introducing oxygen into molten glass havinga low iron content to oxidize the iron in the ferrous state (Fe⁺⁺) toreduce the redox ratio.

2. Discussion of the Presently Available Technology

Solar collectors and solar mirrors use solar energy to heat a fluid,e.g. as disclosed in U.S. Pat. Nos. 4,224,927 and 5,253,105, or toconvert solar energy to electrical energy. In general, the solarcollectors have a cover plate to pass the solar energy, to reduce heatloss due to convection, and to protect the photovoltaic cells of theelectric power generating solar collectors, and the solar mirrors have aglass substrate to pass the solar energy to a reflective coating andreflect the solar energy back through the glass substrate to direct thesolar energy to a designated area. Of particular interest in thefollowing discussion are the glass cover plates and the glasssubstrates.

As is appreciated by those skilled in the art, the glass cover platesused for photovoltaic cover plates, and the glass substrates used forsolar mirrors preferably above 380 nanometers (“nm”) of theelectromagnetic spectrum have a high transmission, e.g. above 90% in thevisible and the infrared (“IR”) range, and a low absorption, e.g. below2% in the visible and the IR ranges. As is appreciated by those skilledin the art, the particular visible and IR range of the electromagneticspectrum, and the peak transmission varies depending on thesemi-conductor material of the photovoltaic cell. For example and notlimiting to the discussion, for a silicon photovoltaic solar cell, thepreferred visible and IR wavelength range is 380-1200 nm, and the peaktransmission is at about 900 nm.

Generally, in the manufacture of flat glass, glass batch materials aremelted; the molten glass is fined and homogenized, and the finedhomogenized molten glass is formed into a flat glass ribbon bycontrollably decreasing the temperature of the molten glass as it floatson a molten metal bath. During the fining of the molten glass, gasbubbles are removed from the molten glass by additions of ingredients tothe batch materials, and/or by moving gases, e.g. carbon monoxide andoxygen through the molten glass, e.g. see U.S. Pat. Nos. 2,330,324 and6,871,514. The batch materials for making glasses having hightransmission, and low absorption, in the visible and the IR range of theelectromagnetic spectrum have no additions of colorants. As isappreciated by those skilled in the art, additions of colorants to thebatch materials have been used to, among other things, reduce thetransmission and increase the absorption in the visible and IR range ofthe subsequently formed glass. Glasses having high visible and IRtransmission are usually referred to as low iron glasses. U.S. Pat. Nos.5,030,593; 5,030,594, and 6,962,887 disclose the making of low ironglasses that are almost colorless by processing raw glass batchmaterials that have a very low content of total iron expressed as Fe₂O₃,e.g. less than 0.020% by weight (hereinafter also referred to as “wt %”or “wt. %”). Iron contents of less than 0.020% by weight (200 parts permillion (hereinafter also referred to as “ppm”)) in batch materials arereferred to as tramp iron because the iron is not added to the batchmaterial but is present as an impurity in the ingredients of the batchmaterial.

Even though the iron content is low in low iron glasses, it is alsopreferred to reduce the weight percent of ferrous iron (Fe⁺⁺) in theglass to maximize the transmission, and minimize the absorption of theglass in the visible and IR range of the electromagnetic spectrum. As isappreciated by those skilled in the art, iron in the ferric state is aless powerful colorant than iron in the ferrous state and shifts thetransmittance spectrum of the glass toward yellow and away from theusual green-blue effect of the ferrous iron in glass. Stated anotherway, increasing iron in the ferric state while decreasing iron in theferrous state, increases the transmission, and decreases the absorptionof the glass in the visible and the IR range. One technique to reducethe weight percent of ferrous iron in the glass is to include ceriumoxide in the glass batch materials because cerium oxide in the glass“decolorizes” the glass. More particularly, cerium oxide is not acolorant in glass, but is a powerful oxidizing agent in glass, and itsfunction in decolorized glass is to oxidize the iron in the ferrousstate (Fe⁺⁺) to iron in the ferric (Fe⁺⁺⁺) state. Although cerium oxideis useful to decolorize the remaining traces of ferrous iron, the use ofcerium oxide has limitations, e.g. but not limiting to the discussion,when the glass is to be used as cover plates for electric powergenerating solar collectors and as glass substrates for solar mirrors.More particularly, exposing low iron glass cover plate having ceriumoxide to the sun has a solarizing effect on the glass, which resultsfrom the photo-oxidation of Ce⁺⁺⁺ to Ce⁺⁺⁺⁺ and the photo-reduction ofFe⁺⁺⁺ to Fe⁺⁺. As is appreciated by those skilled in the art, thesolarization effect of cerium and the photo-reduction of Fe⁺⁺⁺ to Fe⁺⁺reduces the transmission, and increases the absorption, of the glass inthe visible and the IR range of the electromagnetic spectrum, whichreduces the power generation of the solar cells.

As can now be appreciated, it would be advantageous to provide a lowiron glass that has low levels of iron in the ferrous state (Fe⁺⁺) anddoes not have the limitation of the photo-reduction of iron in theferric state (Fe⁺⁺⁺) to iron in the ferrous state (Fe⁺⁺).

SUMMARY OF THE INVENTION

This invention relates to a soda-lime-silica glass, having, among otherthings:

SiO₂ 65-75 weight percent Na₂O 10-20 weight percent CaO 5-15 weightpercent MgO 0-5 weight percent Al₂O₃ 0-5 weight percent K₂O 0-5 weightpercent SO₃ 0-0.30 weight percent Total iron as Fe₂O₃ 0.005-0.120 weightpercent Redox ratio less than 0.550wherein the glass has less than 0.0025 weight percent of CeO₂. Thespectral properties of the glass measured at a thickness 5.5 millimetersinclude, among other things, a visible transmission of greater than 85%measured using C.I.E. standard illuminant “A” with a 2° observer over awavelength range of 380 to 770 nanometers; a total solar infraredtransmittance of greater than 87% measured over a wavelength range of775 to 2125 nanometers, and a total solar energy transmittance ofgreater than 89% measured over a wavelength range of 300 to 2500nanometers, wherein the total solar infrared transmittance and the totalsolar energy transmittance are calculated using Parry Moon air mass 2.0direct solar irradiance data and ASTM air mass 1.5 global solarirradiance data respectively, and integrated using the Rectangular Ruleand Trapezoidal Rule, respectively.

Further, the invention relates to a method of reducing redox ratio ofsoda-lime-silica glass by, among other things, heating a pool of moltensoda-lime-silica glass having iron in a ferrous state (Fe⁺⁺) and in aferric state (Fe⁺⁺⁺), wherein the pool of molten glass is heated with anignited mixture of combustion gas and fuel gas emanating from one ormore burners, wherein flow of the combustion gas exceeds the amount ofcombustion gas required to ignite the fuel gas such that excess oxygenof the combustion gas oxidizes the iron in the ferrous state to iron inthe ferric state to reduce the redox ratio. Optionally oxygen gas cansimultaneously be moved through the pool of molten glass wherein flow ofthe oxygen gas is in a direction from bottom of the pool of molten glassto top of the pool.

Still further, the invention relates to a method of reducing redox ratioof soda-lime-silica glass by, among other things, heating a pool ofmolten soda-lime-silica glass in a heating chamber, the pool of moltenglass having iron in a ferrous state (Fe⁺⁺) and in a ferric state(Fe⁺⁺⁺); moving glass batch materials onto the pool of molten glasscontained in the heating chamber, the batch materials having iron in theferrous state (Fe⁺⁺) and in the ferric state (Fe⁺⁺⁺); melting the glassbatch materials as they float on surface of the molten pool of glass;moving oxygen through the pool of molten glass to oxidize the ferrousiron to the ferric iron to reduce the redox ratio, and forming a glassribbon from the pool of molten glass.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a horizontal section of a glass melting furnace that can beused in the practice of the invention; FIG. 1A is the melting section ofthe furnace, and FIG. 1B is the refining and homogenizing section of thefurnace.

FIG. 2 is a vertical section of the melting section shown in FIG. 1A.

FIG. 3 is an elevated side view partially in cross section of a glassmelting and refining apparatus that can be used in the practice of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

As used herein, spatial or directional terms, such as “inner”, “outer”,“left”, “right”, “up”, “down”, “horizontal”, “vertical”, and the like,relate to the invention as it is shown in the drawing figures. However,it is to be understood that the invention can assume various alternativeorientations and, accordingly, such terms are not to be considered aslimiting. Further, all numbers expressing dimensions, physicalcharacteristics, and so forth, used in the specification and claims areto be understood as being modified in all instances by the term “about”.Accordingly, unless indicated to the contrary, the numerical values setforth in the following specification and claims can vary depending uponthe desired property desired and/or sought to be obtained by the presentinvention. At the very least, and not as an attempt to limit theapplication of the doctrine of equivalents to the scope of the claims,each numerical parameter should at least be construed in light of thenumber of reported significant digits and by applying ordinary roundingtechniques. Moreover, all ranges disclosed herein are to be understoodto encompass any and all subranges subsumed therein. For example, astated range of “1 to 10” should be considered to include any and allsubranges between and inclusive of the minimum value of 1 and themaximum value of 10; that is, all subranges beginning with a minimumvalue of 1 or more and ending with a maximum value of 10 or less, e.g.,1 to 6.7, or 3.2 to 8.1, or 5.5 to 10.

Before discussing several non-limiting embodiments of the invention, itis understood that the invention is not limited in its application tothe details of the particular non-limiting embodiments shown anddiscussed herein since the invention is capable of other embodiments.Further, all documents, such as but not limited to issued patents andpublished patent applications, previously discussed, or referred to, andto be discussed or referred to, herein below are to be considered to be“incorporated by reference” in their entirety. Still further, theterminology used herein to discuss the invention is for the purpose ofdescription and is not of limitation. In addition, unless indicatedotherwise, in the following discussion like numbers refer to likeelements.

Any reference to composition amounts, such as “by weight percent”, “wt%” or “wt. %”, “parts per million” and “ppm” are based on the totalweight of the final glass composition, or the total weight of the mixedingredients, e.g. but not limited to the glass batch materials, whichever the case may be. The “total iron” content of the glass compositionsdisclosed herein is expressed in terms of Fe₂O₃ in accordance withstandard analytical practice, regardless of the form actually present.Likewise, the amount of iron in the ferrous state (Fe⁺⁺) is reported asFeO, even though it may not actually be present in the glass as FeO. Theproportion of the total iron in the ferrous state is used as a measureof the redox state of the glass and is expressed as the ratio FeO/Fe₂O₃,which is the weight percent of iron in the ferrous state (expressed asFeO) divided by the weight percent of total iron (expressed as Fe₂O₃).

The visible range of the electromagnetic spectrum is 380-780 nanometers(hereinafter also referred to as “nm”), and the infra red (hereinafteralso referred to as “IR”) range of the electromagnetic spectrum isgreater than 780 nm and usually considered to be in the range of780-10,000 nm. As used herein, “visible transmittance” or “luminoustransmittance” or “LTA” is measured using C.I.E. standard illuminant “A”with a 2° observer over the wavelength range of 380 to 770 nanometers.Glass color, in terms of dominant wavelength and excitation purity, ismeasured using C.I.E. standard illuminant “C” with a 2° observer,following the procedures established in ASTM E308-90; “total solarinfrared transmittance” or “TSIR” is measured over the wavelength rangeof 775 to 2125 nanometers, and “total solar energy transmittance” or“TSET” is measured over the wavelength range of 300 to 2500 nanometers.The TSIR transmittance data is calculated using Parry Moon air mass 2.0direct solar irradiance data and integrated using the Rectangular Rule,as is known in the art. The TSET transmittance data is calculated usingASTM air mass 1.5 global solar irradiance data and integrated using theTrapezoidal Rule, as is known in the art. Those skilled in the art willunderstand that the above spectral properties, e.g. LTA, infraredtransmission, TSIR and TSET are measured at the actual glass thicknessand can be recalculated at any thickness. In the following discussionthe spectral properties of the glass are given for glasses having astandard thickness of 5.5 millimeter, even though the actual thicknessof a measured glass sample is different than the standard thickness.

The present invention provides a soda-lime-silica glass that is high invisible light and infrared energy transmittance as measured in a normal(i.e. perpendicular) direction to a major surface of the glass sheet,and the glass of the invention is particularly ideal for, but is notlimited to, use as cover plates for electric generating solarcollectors, and glass substrates for solar mirrors. By “high visiblelight transmittance” is meant measured visible light transmittance equalto or greater than 85%, such as equal to or greater than 87%, such asequal to or greater than 90%, at 5.5 mm glass thickness. As isappreciated by those skilled in the art, a glass having a 90% visiblelight transmittance at a thickness of 5.5 mm, has a visible lighttransmission greater than 90% at a thickness less than 5.5 mm and has avisible light transmission less than 90% at a thickness greater than 5.5mm. By “high infrared energy transmittance” is meant measured infraredenergy transmittance equal to or greater than 85%, such as equal to orgreater than 87%, such as equal to or greater than 90%, such as equal toor greater than 91%, at 5.5 mm. As is appreciated by those skilled inthe art, a glass having a 91% infrared energy transmittance at athickness of 5.5 mm, has an infrared energy transmission greater than91% at a thickness less than 5.5 mm and has an infrared visible lighttransmission less than 91% at a thickness greater than 5.5 mm forglasses having a thickness less than 5.5 mm.

The glass of the invention can be made using a conventional non-vacuumrefiner float glass system, e.g. but limited to the type shown in FIGS.1 and 2, or using a vacuum refiner float glass system, e.g. but notlimited to the type shown in FIG. 3. Other types of conventionalnon-vacuum systems are disclosed in U.S. Pat. Nos. 4,354,866; 4,466,562and 4,671,155, and other types of vacuum refiner float glass system aredisclosed in U.S. Pat. Nos. 4,792,536 and 5,030,594.

Referring to FIGS. 1 and 2, there is shown a conventional continuouslyfed, cross-tank fired, glass melting and non-vacuum refining furnace 20having an enclosure formed by a bottom 22, roof 24, and sidewalls 26made of refractory materials. The glass batch materials 28 areintroduced through inlet opening 30 in an extension 32 of the furnace 20known as the fill doghouse in any convenient or usual manner to form ablanket 34 floating on surface 36 of molten glass 38. Overallprogression of the glass as shown in FIGS. 1A and 1B is from left toright in the figures, toward entrance end of a glass forming chamber 40of the type used in the art to make float flat glass.

Flames (not shown) to melt the batch materials 28 and to heat the moltenglass 38 issue from burner ports 42 spaced along the sidewalls 26 (seeFIG. 2) and are directed onto and across the surface 36 of the moltenglass 38. As is known by those skilled in the art, during the first halfof a heating cycle, the flames issue from a nozzle 43 (see FIG. 2) ineach of the ports on one side of the tank 20, as the exhaust of thefurnace moves through the ports on the opposite side of the furnace.During the second half of the heating cycle, the function of the portsare reversed, and the exhaust ports are the firing ports, and the firingports are the exhaust ports. The firing cycle for furnaces of the typeshown in FIGS. 1 and 2 are well known in the art and no furtherdiscussion is deemed necessary. As can be appreciated by those skilledin the art, the invention contemplates using a mixture of air and fuelgas, or a mixture of oxygen and fuel gas, to generate the flames to heatthe batch materials and the molten glass. For a discussion of usingoxygen and fuel gas in the furnace of the type shown in FIG. 1,reference can be made to U.S. patent application Ser. No. 12/031,303filed Feb. 14, 2008 and titled “Use of Photovoltaics for Waste HeatRecovery.”

The glass batch materials 28 as they move downstream from the batchfeeding end or doghouse end wall 46 are melted in the melting section 48of the furnace 20, and the molten glass 38 moves through waist 54 torefining section 56 of the furnace 20. In the refining section 56,bubbles in the molten glass 38 are removed, and the molten glass 38 ismixed or homogenized as the molten glass passes through the refiningsection 56. The molten glass 38 is delivered in any convenient or usualmanner from the refining section 56 onto a pool of molten metal (notshown) contained in the glass-forming chamber 40. As the deliveredmolten glass 38 moves through the glass-forming chamber 40 on the poolof molten metal (not shown), the molten glass is sized and cooled. Adimensionally stable sized glass ribbon (not shown) moves out of theglass-forming chamber 40 into an annealing lehr (not shown). Glassmaking apparatus of the type shown in FIGS. 1 and 2, and of the typediscussed above are well known in the art and no further discussion isdeemed necessary.

Shown in FIG. 3 is continuously fed glass melting and vacuum refiningequipment 78 for melting glass batch materials and refining the moltenglass. Batch materials 80, preferably in a pulverulent state, are fedinto cavity 82 of a liquefying vessel, e.g. a rotating drum 84. A layer86 of the batch material 80 is retained on the interior walls of thevessel 84 aided by the rotation of the drum and serves as an insulatinglining. As the batch material 80 on the surface of the lining 84 isexposed to the heat within the cavity 82, it forms a liquefied layer 88that flows out of a central drain opening 90 at the bottom 92 of thevessel 84 to a dissolving vessel 94 to complete the dissolution ofunmelted particles in the liquefied material coming from the vessel 84.

A valve 96 controls the flow of material from the dissolving vessel 94into a generally cylindrical vertically upright vessel 98 having aninterior ceramic refractory lining (not shown) shrouded in a gas-tight,water-cooled casing 100. A molten stream 102 of refined glass fallsfreely from the bottom of the refining vessel 98 and can be passed to asubsequent stage in the glass making process as detailed in U.S. Pat.No. 4,792,536. For a detailed discussion on the operation of theequipment 78 shown in FIG. 3 reference can be made to U.S. Pat. No.4,792,536.

As is appreciated, the invention is not limited to the process of and/orequipment for making glass, and any of the glass making processes and/orequipment known in the art can be used in the practice of the invention.

Typically, the glass batch used in the glass making apparatus shown inFIGS. 1 and 2 includes sodium sulfate (salt cake) as a melting andrefining aid in the amounts of about 5 to 15 parts by weight per 1000parts by weight of the silica source material (sand), with about 10parts by weight considered desirable to assure adequate refining, i.e.removal of bubbles from the molten glass. The sulfur-containingmaterials can be added such that the retained sulfur content e.g., theaverage amount of SO₃ left in the resultant bulk glass is less than orequal to 0.2 wt. %, such as less than or equal to 0.15 wt. %, such asless than or equal to 0.1 wt. %, such as less than or equal to 0.05 wt.%. In one non-limiting embodiment of the invention, the residual sulfurcan be in the range of 0.005 wt. % to 0.13 wt. %. When operating theglass making apparatus 78 shown in FIG. 3, it is preferred, but notlimiting to the invention, to restrict the sodium sulfate to less thantwo parts by weight per 1000 parts by weight of the silica sourcematerial and to restrict the SO₃ to less than 0.02 wt. %. Moreparticularly, the glass batch materials melted in the glass makingapparatus 78 shown in FIG. 3 are essentially free of sulfur. By“essentially free of sulfur” is meant that no intentional addition ofsulfur-containing compounds is made to the glass batch materials.However, trace amounts of sulfur can be present in the glass due toimpurities in the batch materials or other sources, e.g. but not limitedto cullet. By “trace amounts of sulfur” is meant sulfur in the range ofgreater than 0 wt. % to 0.03 wt. %. The “sulfur” content of the glasscompositions disclosed herein is expressed in terms of SO₃ in accordancewith standard analytical practice, regardless of the form actuallypresent.

Glass batch materials used for making low iron glass cover plates forelectric power generating solar collectors, and for making glasssubstrates for solar mirrors preferably provide a glass that has a highmeasured transmission, e.g. greater than 90%, and a high measured IRtransmission, e.g. greater than 91%. In the practice of the invention,iron is not intentionally added to the batch materials, and iron presentin the molten glass as ferrous iron (Fe⁺⁺) is oxidized to ferric iron(Fe⁺⁺⁺). As is appreciated by those skilled in the art and as discussedabove, additions of CeO₂ are added to the glass batch materials tooxidize the ferrous ion (Fe⁺⁺) to the ferric ion (Fe⁺⁺⁺) to increase thevisible and IR transmission of the glass. It is believed, however, thatexposing glass having CeO₂ to the sun's radiation results insolarization reactions which photo-oxidizes Ce⁺⁺⁺ to Ce⁺⁺⁺⁺ andphoto-reduces Fe⁺⁺⁺ to Fe⁺⁺, which results in the reduction of visibleand IR transmission of the glass. CeO₂ in amounts less than 0.0025 wt. %(25 ppm) or less in the glass does not result in objectionable levels ofsolarization, e.g. a reduction of less than 0.15% of the measuredvisible and IR transmission after exposure to sunlight for 28 days. CeO₂in amounts equal to, or greater than 0.0800 wt. % (800 ppm) results inunacceptable levels of solarization, e.g. a 1.0% reduction in themeasured visible and IR transmission of the glass after exposure tosunlight for 28 days.

In view of the forgoing, in the preferred practice of the inventioningredients that oxidize the ferrous iron Fe⁺⁺to the ferric Fe⁺⁺⁺, andcan be solarized, e.g. CeO₂ are not added to the batch materials, and ifpresent, are present as tramp materials, such that the glass preferablyhas equal to or less than 0.0025 wt. % (25 ppm) CeO₂. Although theinvention is directed to low iron soda-lime-silica glasses, e.g.soda-lime-silica glasses having equal to or less than 0.01 wt. % (100ppm) total iron expressed as Fe₂O₃, the invention is not limitedthereto, and the invention can be practiced to lower the percent byweight of the ferrous iron in high iron glasses, e.g. soda-lime-silicaglasses having greater than 0.01 wt. % (100 ppm) total iron expressed asFe₂O₃. Further, the invention is not limited to glass cover plates forsolar collectors, and to glass substrates for solar mirrors, and can beused (1) as a glass cover plate, or glass substrate for any type ofsolar cell or solar collector; (2) as residential and commercialwindows; (3) as windows for any type of vehicle, e.g. land, air, space,above water, and below water, vehicle; (4) as furniture table tops, and(5) combinations thereof.

Table 1 lists the major constituents and their respective ranges inweight percent of a non-limiting embodiment of a commercial clear floatglass of the invention that can be used to make cover plates for solarcollectors, glass substrates for solar mirrors, and/or commercial,residential and appliance windows.

TABLE 1 CONSTITUENT WEIGHT % SiO₂ 65-75 Na₂O 10-20 CaO  5-15 MgO 0-5Al₂O 0-5 K₂O 0-5 SO3   0-0.30 Total iron as Fe₂O₃ greater than 0-0.120Redox ratio less than 0.350

Usually cerium is added to the batch materials as hydrated ceriumcarbonate (Ce₂CO₃.3H₂0) and can be present in the glass as Ce⁺⁺⁺ (Ce₂O₃)or Ce⁺⁺⁺⁺ (CeO₂). In one non-limiting embodiment of the invention, noCeO₂ is present in the glass. In another non-limiting embodiment of theinvention CeO₂ is present in the glass in amounts equal to or less than0.0025 wt. %. In still another non-limiting embodiment of the invention,CeO₂ can be present in the glass as a tramp material, e.g. as animpurity in the batch materials and/or in the glass cullet added to thebatch materials to aid in the melting of the batch materials. Based onthe forgoing CeO₂ can be present in the glass of the invention withinthe range of 0 to 0.0100 wt. %, preferably in the range of 0 to 0.0075wt. %, more preferably in the range of 0 to 0.0050 wt. %, and mostpreferably in the range of 0 to 0.0025 wt. %.

Clear soda-lime-silica glasses having low amounts of iron have asubstantial absence of color in visible transmittance. In the practiceof one non-limiting embodiment of the invention, the total ironexpressed as Fe₂O₃, is less than about 0.025 wt. % (250 parts permillion), more preferably less than 0.015 wt. % (150 parts per million)and most preferably less than 0.010 wt. % (100 parts per million), andin the preferred practice of the invention the glasses have a redoxvalue (FeO/Fe₂O₃) of less than 0.35, preferably less than 0.25, morepreferably less than 0.20, and most preferably less than 0.150.

Examples of commercial low iron glass that have high measured visibleand IR transmission are presented in Table 2 below.

TABLE 2 (A) (B) CONSTITUENT WEIGHT % WEIGHT % SiO₂ 65-75 65-75 Na₂O10-20 10-20 CaO  5-15  5-15 MgO 0-5 0-5 Al₂O₃ 0-5 0-5 K₂O 0-5 0-5 SO₃0.12-0.20 0.12-0.20 Total iron as Fe₂O₃ 0.005-0.025 0.005-0.025 Redoxratio less than 0.250 less than 0.550 CeO₂  0.18-0.256  0.02-0.100

The glasses of Table 2 can be made using the equipment shown in FIGS.1-3; it should be noted however, that if the equipment shown in FIG. 3is used, the SO₃ is preferably less than 0.02 wt %.

In the practice of the invention, oxygen is introduced into the moltenglass to oxidize the ferrous iron (Fe⁺⁺) to the ferric iron (Fe⁺⁺⁺). Inone non-limiting embodiment of the invention, oxygen is bubbled into thepool of molten glass; in another non-limiting embodiment of theinvention, the ratio of oxygen to fuel or firing gas is increased tooxidize the iron in the ferrous state (Fe⁺⁺) to iron in the ferric state(Fe⁺⁺⁺), and in still another non-limiting embodiment of the invention,oxygen is bubbled into the pool of molten glass and the ratio of oxygento fuel or firing gas is increased to oxidize the iron in the ferrousstate (Fe⁺⁺) to iron in the ferric state (Fe⁺⁺⁺). Support for onenon-limiting embodiment of the invention that oxygen can be used tooxidize the iron in the ferrous state to iron in the ferric state, andfor another non-limiting embodiment of the invention that oxygen can beused to replace all or part of the CeO₂ to oxidize the iron in theferrous state to iron in the ferric state, is provided by the followingexperiment.

Six lab melts were made of low iron glass of the type sold by PPGIndustries, Inc. under the registered trademark Starphire. Each of thelab melts included 1000 grams of Starphire glass cullet. The glasscomposition of the cullet was not analyzed; however, the Starphire glasshas a glass composition within the ranges of the ingredients shown incolumn (B) of Table 2. The cullet was contained in 4-inch silicacrucibles and melted at a temperature of 2600 degrees F. (1427 degreesC.). Oxygen gas was introduced into the molten glass using a porousceramic tube made by etching the bottom 1 inch (2.54 centimeters) of theclosed end of a mullite tube in hydrofluoric acid. Although the sizes ofthe holes were not measured, it is believed the holes had a diameter ofabout less than 1 millimeter.

Sample A was the control sample and no oxygen was introduced into themolten glass of Sample A. The flow rate of oxygen introduced into themolten glass of Sample B was 10 cubic centimeters (“CC”) per minute for30 minutes; into the molten glass of Sample C was 20 CC per minute for30 minutes; into the molten glass of each of Samples D and E was 20 CCper minute for 60 minutes, and into the molten glass of Sample F was 20CC per minute for 120 minutes. Upon conclusion of the introduction ofoxygen of the molten glass of the Samples B-F, it was observed that theends of the tubes in the molten glass of Samples C and D were broken. Itis believed that the tubes broke as a result of thermal shock. Themolten glass of each of the crucibles of Samples A-F was cooled, and theglass analyzed to determine the redox ratio of Sample A (the controlsample) and the redox ratio of the Samples B-F (the “test samples”). TheFeO, Fe₂O₃ and FeO/Fe₂O₃ (the redox ratio) of the Samples A-F are shownin Table 3 below.

TABLE 3 SAMPLE COMPONENT A B C D E F FeO 0.0044 0.0038 0.0022 0.00430.0002 0.0000 Fe₂O₃ 0.0154 0.0162 0.0172 0.0179 0.0172 0.0176 FeO/Fe₂O0.286 0.235 0.128 0.240 0.012 0.000

The Samples B-F each had a lower redox value than the redox value ofSample A indicating that more of the ferrous iron in Samples B-F wasoxidized than in the Sample A. Based on the amount of oxygen added tothe molten glass for sample F and sample C, the efficiency for belowReaction 1 ranged from 0.16 to 0.35%. The efficiency was determined bycalculating the amount of oxygen that reacted with the ferrous irondivided by the total amount of oxygen introduced into the molten glassduring the lab experiment through the porous ceramic tube.

4FeO+O₂

2Fe₂O₃   Reaction 1

As is appreciated by those skilled in the art, the above lab experimentclearly demonstrates that moving oxygen through molten glass oxidizesthe ferrous iron to the ferric iron and lowers the redox ratio.

In the practice of one non-limited embodiment of the invention, theglass batch ingredients selected for making low iron glasses have noadditions of iron, and any iron present in the batch materials ispresent as tramp materials. Iron content generally referred to as trampamounts of iron are amounts of iron less than 0.025 wt. %. For purposesof the present invention, batch materials are selected to have an ironcontent to provide the glass with a total iron expressed as Fe₂O₃ ofless than 0.025 wt. % (250 ppm). As is appreciated by those skilled inthe art, batch materials are selected for minimal iron contamination,but it would be difficult to reduce the total iron content (Fe₂O₃) inthe glass batch materials to provide a glass having less than about0.005 wt. % (50 ppm) without incurring considerable expense. In thenon-limiting embodiment of the invention under discussion, batchselection includes a low iron sand, which can have an iron content ofabout 0.008 wt. % iron (80 ppm) analyzed as Fe₂O₃. Limestone anddolomite, conventional glass batch materials, are avoided because oftheir typical iron contamination. Instead, it is preferred to use apurer source of calcium such as aragonite, which is a mineral form ofcalcium carbonate with only about 0.020 wt. % (200 ppm) Fe₂O₃. Furtherit is preferred to use low iron dolomite, having an iron (Fe₂O₃) contentof less than about 0.020 wt. % (200 ppm). A preferred alumina source isaluminum hydrate, with about 0.008 wt. % (80 ppm) Fe₂O₃. An example of aglass batch mixture that can be used to make glasses within the rangesof the glass of Table 1 is shown in Table 4.

TABLE 4 Batch Constituent Parts by Weight Low Iron Sand 1000 Soda Ash  322-347.8 Aragonite 160-281 Dolomite  0-179 Aluminum hydrate   0-35.1Salt Cake  0-15

As discussed above, in the preferred practice of the invention, ceriumis not added to the batch materials, and preferably, but not limiting tothe invention, cerium is only present as a tramp material, e.g. lessthan 0.010 wt. % (100 ppm).

The batch materials for the glass making processes shown in FIGS. 1-3preferably include the ingredients in the range shown on Table 4, exceptthat the glass making apparatus shown in FIG. 3 is preferably operatedusing two parts by weight of sodium sulfate per 1000 parts by weight ofthe sand (the silica source material); whereas, it is preferred tooperate the glass making apparatus of FIGS. 1 and 2 using 7 parts byweight of sodium sulfate per 1000 parts by weight of the silica sourcematerial. In the practice of the invention, the glass batch materials ofTable 4 provide glasses having compositions shown in Table 5 below.

TABLE 5 (A) (B) (C) Ingredient wt. % wt. % wt. % SiO₂ 72.65 73.26 72.85Na₂O 13.87 15.09 14.04 CaO 10.20 11.03 9.64 MgO 2.94 0.17 3.14 SO₃ 0.1730.196 0.169 Fe₂O₃ 0.0086 0.0087 0.0176 Al₂O₃ 0.04 0.04 0.04 SrO 0.1260.206 0.108

The glass compositions of Table 5 were computer calculated from thebatch formula of Table 4. It should be noted, however, that the glasscomposition of the fifth experiment discussed below was selected to besimilar to computer calculated glass composition of Column (A) of Table5. The computer program does not provide a redox ratio; however, theredox ratios of the invention discussed above are applicable for theglass compositions shown in Table 5. The glasses listed in Table 5 madeusing the glass making apparatus of FIG. 3 would have an SO₃ contentless than 0.02 wt. %. As can be appreciated, the invention is notlimited to the glass compositions listed in Table 5.

Other ingredients having a wt. % less than 0.01 wt. % are trampmaterials which are impurities found in the batch materials and caninclude MnO₂, ZrO₂, CoO, Se, NiO, Cl, P₂O₅, V₂O₅, CeO₂, Cr₂O₃, Li₂O, K₂Oand TiO₂.

The following experiments were conducted on a glass production linehaving a furnace of the type shown in FIGS. 1 and 2 to determine theeffect of exposing molten glass 38 to controlled amounts of O₂ prior tothe molten glass 38 moving through the waist 54 of the furnace 20. Inone experiment two oxygen spargers each consisting of a 2 inch (5.08centimeter (“cm”)) diameter, 6 inch (15.2 cm) long porousAl₂O₃—ZrO₂—SiO₂ refractory (tradename Vision commercially available fromANH Refractories Co.) cylindrical block attached to the end of a 1 inch(2.54 centimeter) diameter and 16 feet (4.9 meters) long water cooledstraight metal pipe were located 3 feet (0.9 meters) from the batchfeeding end 46 of the melter 48 and 4 feet (1.2 meters) from the leftwall of the furnace, and the second sparger was located 3 feet (0.9meters) from the batch feeding end of the melter and 4 feet (1.2 meters)from the right wall of the furnace. Each of the spargers was spaced 42inches (1.1 meters) above the bottom surface of the furnace. Twenty five(25) cubic feet per hour (“CFH”) of oxygen were moved through each ofthe spargers. It was observed that the spargers generated gas bubblesthat were about ⅛ inch (0.32 centimeter) in diameter as they burst onthe surface of the molten glass.

The batch composition had ingredients to make glass similar to the glasslisted in column B of Table 5. The batch ingredients initially added tothe melter did not have any additions of CeO₂, the only CeO₂ present inthe batch materials were tramp amounts, and the CeO₂ present in theglass cullet. Twice during the glass production run hydrated ceriumcarbonate was added to the batch materials. A first sample of the glasswas taken before the first addition of the hydrated cerium carbonate andwas analyzed; the first sample had a redox ratio of 0.48. Three (3)pounds of hydrated cerium carbonate per 1000 pounds of sand was added tothe batch materials for 12 hours. Forty eight (48) hours after the firstaddition of hydrated cerium carbonate, a second sample of the glass wastaken and analyzed; the second sample had a redox ratio of 0.43. TheCeO₂ in the glass increased from 0.04 wt. % to 0.06 wt. %. After aperiod of 6 days after the first addition, a second addition of hydratedcerium carbonate was made. The second addition was 3 pounds of hydratedcerium carbonate per 1000 pounds of sand for 26 hours. Four (4) daysafter the second addition, a third sample of the glass was taken andanalyzed. The third sample of glass had a redox ratio of 0.471;contained 0.0102 wt % (102 ppm) Fe₂O₃, and 0.04 wt % (400 ppm) CeO₂. Theusual level of CeO₂ is about 0.07% (700 ppm) and the usual level of theredox ratio is in the range of about 0.48-0.50. The results from thefirst experiment suggested that the introduction of oxygen gas into themolten glass through the two porous refractory spargers can serve as asubstitute for adding CeO₂ to oxidize the ferrous iron to the ferriciron, and to lower the glass redox ratio by about 0.01-0.03, in a largecommercial glass furnace.

A second experiment was conducted on a glass production run to makeclear glass having 0.10 wt % Fe₂O₃, i.e. high iron glass. In the secondexperiment, the sparger positions in relationship to the furnace wallswas the same, however the spargers were spaced 8 inches (20 cm) from thebottom surface of the furnace. Further, each of the the spargers in thesecond experiment was a thicker porous refractory cylindrical block (3inch (7.6 cm) diameter compared to only 2 inch (5.08 cm) diameter usedin the first experiment) to increase the useable life of the spargers.The oxygen flow rate was 20 CFH at 40 PSI through each of the spargers.The average redox ratio of the glass two weeks before oxygen was flowedthrough the spargers was 0.338 and the range of the redox ratio was0.005. The average redox ratio with oxygen moving through the spargerswas 0.336 and the range of the redox ratio was 0.01. There was nosignificant change in the mean value of redox ratio, only an increase inthe variability of the redox value. The conclusion of the secondexperiment was that while the glass redox ratio was lowered at leastpart of the time while using the oxygen spargers, the glass redox ratiowas not lowered on a continuous basis due to non-homogeneous mixing ofthe molten glass in the furnace.

In a third experiment, the production run was making a glass compositionincluded 0.05 wt % CeO₂. In the third experiment, oxygen was movedthrough selected bubblers of one row of 19 individual gas bubblers(water cooled metal tubes) 150 (see FIG. 1A) mounted in the base 26 ofthe furnace 20. The bubblers extended upward into the molten glass about24 inches (0.61 meters) from the bottom surface of the furnace and 33inches (0.84 meters) below the surface 36 of the molten glass 38. Thebubblers 150 were positioned about 50 feet from the wall 46 of thefurnace 20 in the area of the 4^(th) port 42 (see FIG. 2). The bubblers150 were spaced about 18 inches (0.46 meters) apart and span the furnace20 in a perpendicular fashion to the direction of the molten glass flow.Initially oxygen was moved through 6 bubblers, and then over the nextthree days through 12 of the remaining 13 bubblers; one bubbler did notfunction because it was clogged. Although the position of the first sixbubblers was not recorded, it is believed the six bubblers were thethree outer bubbles on each end of the row of bubblers. The oxygen flowwas initially 5 CFH through each of the 18 bubblers and was increasedafter 3 days by 5 CFH, and increased by 5 CFH once again 4 days afterthe first increase. The last step of 5 CFH was reversed because the highrate of oxygen bubbling was entraining and leaving residual bubbles inthe molten glass. It was observed that the bubblers generated gasbubbles that were about 6 inches (15.2 cm) in diameter as they burst onthe surface of the molten glass. The glass redox ratio prior tointroducing oxygen gas through the bubblers was 0.45. The glass madewith oxygen moving through the 18 bubblers and after the last step of 5CFH was reversed had a redox ratio of 0.41 and an Fe₂O₃ of 0.0096 wt. %.The use of the oxygen gas in the bubblers lowered the glass redox by0.04.

A fourth experiment was conducted on the glass composition of the thirdexperiment except that the only CeO₂ present in the batch materials wastramp CeO₂ in the glass cullet in an amount of 0.04 wt. %. In the fourthexperiment, the bubblers were raised to a position 27 inches (0.69meters) from the level of the molten glass and the oxygen was movedthrough each of the 18 bubblers 150 at a flow rate of 12.5 CFH. Theoxygen gas flow rate was increased from 12.5 CFH to 17.5 CFH perbubbler, and from 17.5 CFH to 20 CFH per bubbler over the next fivedays. The rate of oxygen was dropped back to 17.5 CFH because the highrate of oxygen gas bubbling was entraining and leaving residual bubblesin the molten glass. A sample of the glass while bubbling oxygen gas ata flow rate of 17.5 CFH per bubbler had a redox ratio of 0.467, 0.0092wt. % (92 ppm) Fe₂O₃ and 0.033 wt. % CeO₂ (330 ppm). It is believed thatbubbling oxygen gas at a total flow rate of 100 CFH into 7564 cubic feetof molten glass for 24 hours (2400 CF of oxygen per 7564 cubic feet ofmolten glass) is equal to about 0.01 wt. % CeO₂ in terms of causing anequivalent decrease in the glass redox ratio. The efficiency of bubblingwith oxygen gas in the commercial glass furnace was calculated and isabout 0.12%, which is similar to that observed in the laboratoryexperiment. The efficiency was determined by calculating the amount ofoxygen that reacted with the ferrous iron divided by the total amount ofoxygen introduced into the molten glass during the fourth experimentthrough the 18 bubblers 150.

From the above experiments it was concluded that the glass redox ratiocan be lowered by introducing oxygen gas into the molten glass as asubstitute for the need to add CeO₂ to oxidize the iron in the ferrousstate (Fe⁺⁺) to iron in the ferric state (Fe⁺⁺⁺). The oxygen gas can beintroduced through either a sparger consisting of a porous refractoryblock or a water cooled metal bubblers. It was observed that the size ofthe bubbles generated by the oxygen gas was much smaller using thesparger than with the water cooled bubbler More particularly, the sizeof the bubbles from the spargers were similar to the bubbles movedthrough the molten glass in the lab experiment. In the instance when theglass is made in the glass making apparatus shown in FIG. 3, the oxygenwould be bubbled into the molten glass in the dissolution chamber 94through bubblers 110 (only one shown in FIG. 3) mounted through the base112 of the dissolution chamber 94.

With reference to FIG. 2, in another non-limiting embodiment of theinvention, oxygen to oxidize the ferrous iron (Fe⁺⁺) to ferric iron(Fe⁺⁺⁺) is provided by increasing the ratio of combustion air, i.e.oxygen gas to the fuel or firing gas at the firing ports. The normalfiring ratio of combustion air to fuel gas is 10.9 as determined by theformula “total combustion air flow rate (the combustion air to all ofthe firing ports) divided by total fuel gas flow rate (fuel gas to allof the firing ports).” As is appreciated by those skilled in the art,the flow rate of combustion air and fuel gas is not evenly distributedto each of the firing ports; however, in the practice of the inventionthe total flow rate of the combustion air and the total flow rate of thefuel gas is of interest. Further, as is appreciated by those skilled inthe art, the combustion gas includes 21% oxygen and the remainingpercent mostly nitrogen. Therefore, the normal firing ratio of oxygen tofuel gas for combustion air/fuel gas fired furnaces is 2.29 (10.9 totalcombustion air/total fuel gas×0.21 oxygen in combustion air). In thefollowing discussion, the “air firing ratio” is determined by theformula “total combustion air flow rate (the combustion air to all ofthe firing ports) divided by total fuel gas flow rate (fuel gas to allof the firing ports)” and is normally 10.9. The “oxygen firing ratio”for an oxygen/fuel gas fired furnace is determined by the formula “totaloxygen gas flow rate (the oxygen to all of the firing ports) divided bytotal fuel gas flow rate (fuel gas to all of the firing ports)” and isnormally 2.29, and the “oxygen firing ratio” for a combustion air/fuelgas firing furnace is determined by the formula “total combustion airflow rate times percent of oxygen in the combustion air divided by totalfuel gas flow rate (fuel gas to all of the firing ports)” and isnormally 2.29. Increasing the air firing ratio to greater than 11.0, orthe oxygen firing ratio to 2.31 by increasing the total combustion airflow rate or the total combustion oxygen, respectively, provides excessoxygen to oxidize the ferrous iron (Fe⁺⁺) to ferric iron (Fe⁺⁺⁺).

In a fifth experiment that was conducted on a commercial glass furnacemaking low iron glass having a glass composition similar to the computercalculated glass composition of column A in Table 5. A sample of glasswas taken and analyzed; the glass had a redox ratio of 0.45. During thefifth experiment, oxygen gas at a flow rate of 3 CFH per bubbler wasmoved through the 18 bubblers 150 located in Port 4 of the glass furnace20. The batch materials were changed by using low iron dolomite toreplace part of the aragonite in the glass batch. The dolomite increasesthe MgO content of the glass, which increases the durability of theglass as is known in the art. It is believed that the addition ofdolomite also helps to lower the glass redox, because the dolomite doesnot contain high levels of carbon impurities, which are present in thearagonite and can act as a reducing agent to reduce the ferric iron(Fe⁺⁺⁺) to the ferrous iron (Fe⁺⁺).

Combustion air at each of the 7 ports 42 on each side of the furnace 20was increased during their firing cycle by increasing the air firingratio from 12.3 to 13.3 in steps of 0.1-0.4 (increasing the oxygenfiring ratio from 2.58 to 2.79 in steps of 0.02-0.084) each over a fiveday period. About 72 hours after the ratio was increased, a sample ofglass was taken and analyzed. The redox ratio of the sample was 0.39.The low iron float glass composition produced is similar to the computergenerated glass composition of Column (A) in Table 5 and contained0.0084 wt. % (84 ppm) Fe₂O₃ and 0.0021 wt. % (21 ppm) CeO₂. The glasshad a LTA (visible transmittance value) of 91.3%, a TSIR value of 90.4%and a TSET value of 90.7% at an actual thickness of about 3.2 mm (0.1254inches). An LTA value of 91.3% is a very high glass transmittance thatis useful as a cover plate to protect the photovoltaic cells in electricpower generating solar collectors and as a glass substrate for solarmirrors. It is concluded from this fifth experiment that the glass redoxratio can be lowered by about 0.06 by increasing the air firing ratio(the oxygen firing ratio).

As is appreciated by those skilled in the art, increasing the oxygenfiring ratio and operating the furnace at elevated temperatures canincrease NOx emissions. This can be managed by reducing the temperatureof the furnace and/or by appropriate emission control equipment. Theinvention is not limited to operating temperature of the furnace and/orby the use of emission control systems.

From the above it can be appreciated that increasing the air firingratio (the oxygen firing ratio) provides oxygen to the molten glass tooxidize the ferrous iron (Fe⁺⁺) to ferric iron (Fe⁺⁺⁺). As can beappreciated, the invention is not limited to any particular ratio value;however, it is preferred to have an oxygen firing ratio of 2.31 (an airfiring ratio of 11.0), more preferred an oxygen firing ratio of 2.63 (anair firing ratio of 12.5), and most preferred an oxygen firing ratio of2.71 (an air firing ratio of 12.9). Further as can be appreciated,bubbling oxygen through the molten glass provides oxygen to the moltenglass to oxidize the ferrous iron (Fe⁺⁺) to ferric iron (Fe⁺⁺⁺). In onenon-limiting embodiment of the invention, and as discussed above, 2400CF per 24 hours of oxygen per 7564 cubic feet of molten glass (0.32CFper 24 hours per cubic foot of molten glass) is equal to about 0.01%CeO₂ in terms of causing an equivalent decrease in the glass redoxratio. Still further, as can be appreciated, increasing the air firingratio (the oxygen firing ratio) while bubbling oxygen through the moltenglass increases the amount of oxygen to the molten glass to oxidize theferrous iron (Fe⁺⁺) to ferric iron (Fe⁺⁺⁺) and can be used to avoidexcessive increases of the air firing ratio (the oxygen firing ratio)thereby reducing environmental concerns.

Based on the forgoing, the invention can be practiced to make a glassfor solar control cover plates and for solar mirrors, e.g. low ironglass having the components in the range shown in Table 6, and theproperties discussed below.

TABLE 6 COMPONENT RANGE SiO₂ 65-75 wt. % Na₂O 10-20 wt. % CaO 5-15 wt. %MgO greater than 0 to 5 wt. % CeO₂ less than 0.0025 wt. % SO₃ 0.12-0.2wt. % Fe₂O₃ (total iron) equal to or less than 0.01 wt. % Redox ratioless than 0.400, or less than 0.350, or less than 0.200, or less than0.150

The glasses of Table 6 at a glass thickness of 5.5 millimeters have anLTA equal to or greater than 85%, or equal to or greater than 87%, orequal to or greater than 90%; a TSIR equal to or greater than 85%, orequal to or greater than 87%, or equal to or greater than 90%, or equalto or greater than 91%, and a TSET equal to or greater than 89%, orequal to or greater than 90%, or equal to or greater than 91%. Thespectral properties of the glass vary as the redox ratio and/or theFe₂O₃ (total iron) vary as was discussed above.

Further, based on the forgoing, the invention can be practiced to make aglass for commercial and residential buildings, furniture andappliances, and for land, above and below water, and aerospace, e.g.high iron glass having the components in the range shown in Table 7, andthe properties discussed below.

TABLE 7 COMPONENT RANGE SiO₂ 65-75 wt. % Na₂O 10-20 wt. % CaO 5-15 wt. %MgO greater than 0 to 5 wt. % CeO₂ less than 0.080 wt. %, or less than0.060 wt. %, or less than 0.030 wt. % or less than 0.020 wt. %, or lessthan 0.010 wt. % SO₃ 0.12-0.2 wt. % Fe₂O₃ (total iron) greater than 0.01wt. % to 0.12 wt. % Redox ratio less than 0.550, or less than 0.400, orless than 0.350, or less than 0.200, or less than 0.150

The glasses of Table 7 at a glass thickness of 5.5 millimeters, have anLTA equal to or greater than 85%, or equal to or greater than 87%, orequal to or greater than 90%; a TSIR equal to or greater than 85%, orequal to or greater than 87%, or equal to or greater than 89%, or equalto or greater than 90%, and a TSET equal to or greater than 88%, orequal to or greater than 89%, or equal to or greater than 90%. Thespectral properties of the glass vary as the redox ratio and/or theFe₂O₃ (total iron) vary as was discussed above.

The above glasses are preferably, but not limited to the invention, madein glass making equipment similar to, but not limited to the type shownin FIGS. 1 and 2. The above glass can be made in glass making equipmenthaving a vacuum refiner, e.g. similar to, but not limited to the typeshown in FIG. 3 by reducing the SO₃ to less than 0.010 wt % as discussedabove.

It will be readily appreciated by those skilled in the art thatmodifications can be made to the invention without departing from theconcepts disclosed in the foregoing description. Accordingly, theparticular embodiments described in detail herein are illustrative onlyand are not limiting to the scope of the invention, which is to be giventhe full breadth of the appended claims and any and all equivalentsthereof.

1. A soda-lime-silica glass, comprising: SiO₂ 65-75 weight percent Na₂O10-20 weight percent CaO 5-15 weight percent MgO 0-5 weight percentAl₂O₃ 0-5 weight percent K₂O 0-5 weight percent SO₃ 0-0.30 weightpercent Total iron as Fe₂O₃ 0.005-0.120 weight percent Redox ratio lessthan 0.550

wherein the glass has less than 0.0025 weight percent of CeO₂ andspectral properties of the glass measured at a thickness 5.5 millimeterscomprises: a visible transmission of greater than 85% measured usingC.I.E. standard illuminant “A” with a 2° observer over a wavelengthrange of 380 to 770 nanometers; a total solar infrared transmittance ofgreater than 87% measured over a wavelength range of 775 to 2125nanometers, and a total solar energy transmittance of greater than 89%measured over a wavelength range of 300 to 2500 nanometers, wherein thetotal solar infrared transmittance and the total solar energytransmittance are calculated using Parry Moon air mass 2.0 direct solarirradiance data and ASTM air mass 1.5 global solar irradiance datarespectively, and integrated using the Rectangular Rule and TrapezoidalRule, respectively.
 2. The glass according to claim 1 wherein thespectral properties comprise: the visible transmission is greater than87%; the total solar infrared transmittance of greater than 89%, and thetotal solar energy transmittance of greater than 90%.
 3. The glassaccording to claim 1 wherein the spectral properties comprise: thevisible transmission is greater than 90%, and the total solar infraredtransmittance of greater than 90%.
 4. The glass according to claim 3,wherein the total iron as Fe₂O₃ is 0.005-0.025 weight percent and theredox ratio is less than 0.350.
 5. The glass according to claim 1,wherein the total iron as Fe₂O₃ is 0.005-0.025 weight percent and theredox ratio is less than 0.200.
 6. A method of reducing redox ratio ofsoda-lime-silica glass comprising: heating a pool of moltensoda-lime-silica glass having iron in a ferrous state (Fe⁺⁺) and in aferric state (Fe⁺⁺⁺), wherein the pool of molten glass is heated with anignited mixture of combustion gas and fuel gas emanating from one ormore burners, wherein flow of the combustion gas exceeds the amount ofcombustion gas required to ignite the fuel gas such that excess oxygenof the combustion gas oxidizes the iron in the ferrous state to iron inthe ferric state to reduce the redox ratio.
 7. The method according toclaim 6 wherein oxygen firing ratio of the ignited mixture is greaterthan 2.31, and the oxygen fuel ratio is determined as follows: totalflow of the combustion gas to all of the burners times the percent ofoxygen in the combustion gas divided by total flow of the fuel gas toall of the burners.
 8. The method according to claim 7 wherein theoxygen firing ratio is in the range of 2.31-2.71.
 9. The methodaccording to claim 7 wherein the oxygen firing ratio is greater than2.63.
 10. The method according to claim 8 wherein the oxygen firingratio is greater than 2.71.
 11. The method according to claim 6 whereinthe molten glass further comprises greater than zero and less than0.0800 weight percent of CeO₂.
 12. The method according to claim 6wherein the CeO₂ is less than 0.0025 weight percent.
 13. The methodaccording to claim 6 wherein the CeO₂ is in the range of greater thanzero and equal to or less than 0.0025 weight percent and the redox ratiois equal to or less than 0.350.
 14. The method according to claim 6further comprising controllably cooling portions of the pool of moltenglass to provide a glass wherein the glass has total iron as Fe₂O₃ inthe range of 0.005-0.120 weight percent, a redox ratio of less than0.550, wherein the glass has less than 0.0025 weight percent of CeO₂ andspectral properties of the glass measured at a thickness 5.5 millimeterscomprises: a visible transmission of greater than 85% measured usingC.I.E. standard illuminant “A” with a 2° observer over a wavelengthrange of 380 to 770 nanometers; a total solar infrared transmittance ofgreater than 87% measured over a wavelength range of 775 to 2125nanometers, and a total solar energy transmittance of greater than 89%measured over a wavelength range of 300 to 2500 nanometers, wherein thetotal solar infrared transmittance and the total solar energytransmittance are calculated using Parry Moon air mass 2.0 direct solarirradiance data and ASTM air mass 1.5 global solar irradiance datarespectively, and integrated using the Rectangular Rule and TrapezoidalRule, respectively.
 15. The method according to claim 14 wherein thespectral properties comprise: the visible transmission is greater than87%; the total solar infrared transmittance of greater than 89%, and thetotal solar energy transmittance of greater than 90%.
 16. The methodaccording to claim 15 wherein the spectral properties comprise: thevisible transmission is greater than 90%, and the total solar infraredtransmittance of greater than 90%.
 17. The method according to claim 16,wherein the total iron as Fe₂O₃ is 0.005-0.025 weight percent and theredox ratio is less than 0.350.
 18. The method according to claim 6further comprising moving oxygen gas through the pool of molten glasswherein flow of the oxygen gas is in a direction from bottom of the poolof molten glass to top of the pool.
 19. A method of reducing redox ratioof soda-lime-silica glass comprising: heating a pool of moltensoda-lime-silica glass in a heating chamber, the pool of molten glasshaving iron in a ferrous state (Fe⁺⁺) and in a ferric state (Fe⁺⁺⁺);moving glass batch materials onto the pool of molten glass contained inthe heating chamber, the batch materials having iron in the ferrousstate (Fe⁺⁺) and in the ferric state (Fe⁺⁺⁺); melting the glass batchmaterials as they float on surface of the molten pool of glass; movingoxygen through the pool of molten glass to oxidize the ferrous iron tothe ferric iron to reduce the redox ratio, and forming a glass ribbonfrom the pool of molten glass.
 20. The method according to claim 19further comprising controllably cooling potions of the pool of moltenglass to provide a glass wherein the glass has total iron as Fe₂O₃ inthe range of 0.005-0.120 weight percent, a redox ratio of less than0.550, wherein the glass has less than 0.0025 weight percent of CeO₂ andspectral properties of the glass measured at a thickness 5.5 millimeterscomprises: a visible transmission of greater than 85% measured usingC.I.E. standard illuminant “A” with a 2° observer over a wavelengthrange of 380 to 770 nanometers; a total solar infrared transmittance ofgreater than 87% measured over a wavelength range of 775 to 2125nanometers, and a total solar energy transmittance of greater than 89%measured over a wavelength range of 300 to 2500 nanometers, wherein thetotal solar infrared transmittance and the total solar energytransmittance are calculated using Parry Moon air mass 2.0 direct solarirradiance data and ASTM air mass 1.5 global solar irradiance datarespectively, and integrated using the Rectangular Rule and TrapezoidalRule, respectively.
 21. The method according to claim 20 wherein thespectral properties comprise: the visible transmission is greater than90%; the total solar infrared transmittance of greater than 90%, and thetotal solar energy transmittance of greater than 90%, and the total ironas Fe₂O₃ is 0.005-0.025 weight percent and the redox ratio is less than0.350.
 22. The method according to claim 19 wherein the batch materialsare melted as the batch materials float on surface of the pool of moltenglass and moving the oxygen gas bubbles is accomplished by moving aplurality of spaced streams of oxygen gas bubbles upward through thepool of molten glass toward the surface of the pool of molten glass froma position below the surface of the pool of molten glass and downstreamfrom the melting batch materials, wherein the streams of gas bubbles arein a line transverse to direction or flow of the molten glass.